Extenuating circumstances are different for each person. They can be a one time thing, like the two-week long family vacation your mom planned months ago, or something with a long-term impact, like the fact that you observe the Sabbath, are at the beginning of a pregnancy, or have a medical condition that requires you to take more time off than the average employee. What these issues always have in common is that they can cause a great level of stress -- you want the job, but you have this other major life thing that you can’t sacrifice either, and it’s easy to worry yourself into a tizzy trying to figure out when to be forthcoming.
We recommend waiting to share extenuating circumstances until you have an offer on the table, unless the topic comes up naturally. Sure, the potential employer might be blindsided for a second, but they legally can’t rescind an offer for things like religious practices or disabilities if there are reasonable accommodations that can be made that allow you to do the job. But if you mention your needs in an interview, you run the risk of losing the offer, and it's nearly impossible to prove discrimination occurred. So withhold this information until you are offered the position, but carefully prepare the statement you make to your future boss to start things off on the right foot.
For something like a vacation, it’s as simple as saying, “Thanks for the offer! I’m curious about the company’s vacation policy -- my family is going to Europe for two weeks next month to celebrate my mom’s birthday, and it’s been planned forever. Would it be possible to take that time off?” Sometimes, the answer will be “Sure thing!” In other instances, you might have to cancel or take the time off unpaid. In that case, it’s your call if the trip or the money is more important.
For something recurring, it’s a little tougher. You want to make sure you have proof you can do the job with reasonable accommodations. Try something like, “Thanks for the offer. I want to let you know that I observe many religious holidays that require me to be out of the office for several extra days of the year. I can send you a schedule of the exact dates, and I’m prepared to make up the hours by staying later during the other days of those weeks. If you’d like to talk to some past employers about how this has worked, I’m happy to provide references.” For a medical condition, it may be more sensitive to discuss, but do your best to explain your medical needs without oversharing. For instance, “Thanks for the offer. This is a little awkward, but I think I should let you know that I have a medical condition that requires frequent visits to the doctor's office and sometimes interferes with my ability to come into the office. I may need to take extra time off for doctor’s appointments and work from home on certain days. Is that something you can accommodate? I can assure you this won't impact the quality of my work, and I’m happy to provide references from previous employers who can attest to my work ethic.”
It’s possible, if you got the job through a referral, that your potential employer already knows this information and is comfortable with it. Sometimes they’ll even bring it up in the interview. But more often, it’s brand new information and may throw them for a loop. Monitor their reactions -- if they express understanding with their surprise, great! Remember to be ten times better at your job than the other employees, and you’ll succeed in the role. But if they start howling through the phone about how you’re incredibly shady and can’t believe you trapped them like this, decline the offer. At that point, you’re signing yourself up for constant abuse, and that’s never worth it. Aren’t you glad you know? Your sucky medical condition may have saved you from working for an insufferable maniac!